Field of the Invention
The present invention relates to a subject information acquisition device for receiving elastic waves generated inside the subject, and acquiring subject information.
Description of the Related Art
As one of in vivo imaging techniques using near-infrared light, photo acoustic imaging (PAI) is known. General principles of photo acoustic imaging will be explained below in detail.
By irradiating a subject, such as a living body, with pulsed light generated from a light source, light which is propagated and diffused within the subject is absorbed with a light absorption body, and photo acoustic waves are generated. The principle of generation of the photo acoustic waves is known as the photo acoustic effect. In a living body, not all tissues absorb light in the same manner. A tumor's absorption of light energy in near-infrared light is higher than surrounding tissues. Therefore, a tumor that absorbs more light than surrounding tissues, instantly expands, and generates photo acoustic waves.
In photo acoustic imaging, the photo acoustic waves are received by a transducer, and by signal-processing the received signal (image reconstruction), information such as spatial distribution of the initial sound pressure of photo acoustic waves that occur upon absorption of light energy in the subject is turned into images. The spatial distribution of sound pressure generated is related to an absorption coefficient of light. Therefore, diagnosing a subject using a spatial distribution related to the absorption coefficient of light is being studied.
As an example of photo acoustic imaging, Non-Patent Literature 1 (Minghua Xu and Lihong V. Wang, “Universal back-projection algorithm for photoacoustic computed tomography”, PHYSICAL REVIEW E 71, 016706 (2005)) discloses a photo acoustic imaging technique which calculates initial sound pressure of photo acoustic waves by image reconstruction by time-domain back projection.
Hereinafter, the image reconstruction method disclosed in Non-Patent literature 1 will be explained with reference to FIG. 8. FIG. 8 illustrates a block diagram in which an array transducer 1300 receives photo acoustic waves generated in an interest region 1110 within a subject 1100. The array transducer 1300 includes n transducers 1301 to 130n arranged one-dimensionally.
Non-Patent Literature 1 discloses to acquire the initial sound pressure p0 in the interest region 1110 by the time-domain back projection represented by formula (1).
                                          p            0                    ⁡                      (            r            )                          =                              ∑                          i              =              1                        n                    ⁢                                    ΔΩ              i                        ×                                          b                (                                                      d                    i                                    ,                                                                                                                                    d                          i                                                -                        r                                                                                    v                                                  )                            /                                                ∑                                      i                    =                    1                                    n                                ⁢                                  ΔΩ                  i                                                                                        Formula        ⁢                                  ⁢                  (          1          )                    
In formula (1), di represents a position vector of a transducer 130i, r represents a position vector of the interest region 1110, v represents the sound velocity within the subject 1100, and b represents a reception signal that has been subjected to a process including differentiation processing. A sum symbol Σ represents the sum of the transducers 1301 to 130n. Furthermore, (|di−r|/v) indicates a time t at which the photo acoustic waves generated in the interest region 1110 arrive at the transducer 130i. 
Furthermore, the reception signal b which has been subjected to differentiation processing or the like is represented by formula (2).b(di,t)=2·pd(di,t)−2·t·∂pd(di,t)/∂t  Formula (2)
In formula (2), p(di, t) represents a value at the time t of the reception signal output by the transducer located at the position di.
Furthermore, ΔΩi in formula (1) represents a weighting factor represented by formula (3).
                              ΔΩ          i                =                                                            Δ                ⁢                                                                  ⁢                                  S                  i                                                                                                                    r                    -                                          d                      i                                                                                        2                                      ·                          [                                                n                                      0                    ⁢                    i                                    s                                ·                                                      r                    -                                          d                      i                                                                                                                      r                      -                                              d                        i                                                                                                                            ]                                =                                                                      Δ                  ⁢                                                                          ⁢                                      S                    i                                                                                                                                  r                      -                                              d                        i                                                                                                  2                                            ·              cos                        ⁢                                                  ⁢                          θ              i                                                          Formula        ⁢                                  ⁢                  (          3          )                    
In formula (3), n0is represents a unit normal vector of a reception surface of the transducer 130i, θi represents an angle formed by a vector r−di and the vector n0is, and ΔSi represents the area of a reception surface of the i-th transducer. That is, the weighting factor ΔΩi is equal to the solid angle of a cone whose vertex is the interest region 1110 and whose bottom surface is the reception surface of the transducer 130i. 
As described above, Non-Patent Literature 1 discloses to obtain initial sound pressure by correcting a reception signal using a weighting factor based on the solid angle formed by an interest region and a transducer.
Furthermore, Patent Literature 1 (Japanese Patent Laid-Open No. 2010-104816) discloses photo acoustic imaging that receives photo acoustic waves in a wide region of a subject by mechanically moving a transducer. However, in the case where the calculation of initial sound pressure is performed by using many reception signals obtained in the wide region of the subject, the amount of data to be handled increases, therefore it takes a long time to calculate the initial sound pressure.
Thus, Patent Literature 1 discloses a method to sequentially obtain initial sound pressures, using signals received up to the point of calculation during the scanning. Furthermore, Patent Literature 1 discloses to add a plurality of initial sound pressures obtained sequentially.